It is well-known to build commutators utilizing an annular shell and a clamping ring to clamp individual commutator bar5therebetween. Clamping force is generally provided by a set of studs which extend through alined passages in the ring and the shell and to which a preset torque is applied thereby to resiliently clamp the commutator bars in position by forcing the clamping ring towards the shell.
In many cases these studs are simply bolt-like elements designed with threads at either end and a necked-down portion in the center so that prestressing can predict the actual tension on the stud and thus the pressure exerted via the clamping ring on the bars forcing them towards the shell. Such a simple structure is normally applied on narrow commutators i.e. commutators having lengths in the axial direction of say about a foot to about two feet.
When the commutator length or motor speed is increased significantly special studs are used that are supported along their length to prevent deflection and vibration. This support is normally obtained by putting a ring with accurate holes therethrough extending in the axial direction positioned to snugly receive a rigid boss generally at about the mid length of each stud so that the effective length of the stud for vibration purposes is reduced to one half.
Recently a failure has been experienced in a particular installation incorporating a relatively short commutator having a simple stud design with no intermediate support. In this particular installation the motor was operated at low speed under high impact loading in a steel mill to drive the rougher rolls. Surprisingly it was found that the studs after a period of time fractured and upon close examination it was found that the fracture was due to fatigue. Upon testing it was found that the commutator stud assembly had exceptionally low damping which can have serious consequences in that it can result in stresses, generated by a very high percentage of the vibrations, of sufficient magnitude to diminish stud life. This had not been evident based on conventional design criteria used to produce such commutator.